1. Field of the Invention
This invention relates to a laser beam machining method and a device applicable thereto which carry out a cutting and welding, etc. of a workpiece made of metal by use of a laser beam, particularly to a laser beam machining method and a device that perform machining under actual machining conditions after having machined the workpiece under preliminary machining conditions so as to improve a surface state of the workpiece.
2. Description of Background Art
Generally, in a laser beam machining of a workpiece made of metal such as carbon steel, stainless steel or aluminum, a laser beam is irradiated onto the workpiece while injecting an assist gas thereto. When a workpiece having a surface coated with a substance of low melting point, an evaporating substance of low melting point possibly intrudes into a working area during the machining operation. In this case, a work quality is deteriorated.
FIGS. 21 to 26 concern conventional cutting and welding techniques which attempt to solve this problem. In FIGS. 22-25, arrow 101 shows a machining direction or cutting direction by the laser beam 1.
One way to cope with the above disadvantage is disclosed in Japanese Patent Publication (Kokai) No. 4-333386, in which an assist gas is used when machining a material coated with a substance of low melting point by the laser beam, as shown in FIG. 21. This art intends to restrain evaporation of the low melting point substance and improve the quality of the machined product.
In FIG. 21, a laser beam 1 is radiated from a laser oscillator (not shown) and focused by a lens 2 onto a workpiece 3a or a galvanized iron sheet. The workpiece 3a has a surface covered with a galvanized layer 3b. A pair of gas bombs 4a and 4b contain therein oxygen (O.sub.2) and argon (Ar), respectively, and supplies the gasses to a mixer 6. A machining head 7 has a nozzle 8 at its leading end so that it permits a mixed gas of oxygen and argon to be injected from the nozzle 8 toward a machining point 9 on the workpiece 3a.
An operation of the above device will be described hereunder.
The laser beam 1 going out of the laser oscillator is led to the machining head 7 by a bend mirror (not shown). Then, the laser beam 1 is focused by the lens 2 and irradiated from the nozzle 8 onto the machining point 9 of the galvanized layer 3b of the workpiece 3a. An energy density of the laser beam 1 at the galvanized layer 3b is changed according to the kind, the plate thickness or the machining speed of the workpiece 3a. Moreover, oxygen gas and argon gas are fed from the bombs 4a and 4b and mixed in the mixer 6. The mixed gas is conducted to below the condenser lens 2 in the machining head 7 and injected from the nozzle 8 onto the galvanized layer 3b along with the laser beam 1. The mixed gas is used to oxidize the zinc of the galvanized iron sheet, by the oxygen gas therein, into a zinc oxide or a zinc peroxide. As a result, the zinc does not evaporate and spatter is lessened, thereby enabling laser beam welding with less blowholes.
A technique disclosed in a Japanese Patent Publication (Kokai) No. 4-138888 divides a laser beam into two beams so that one beam peels a substance of low melting point and the other beam performs welding, as shown in FIG. 23.
In FIG. 23, the device has a laser oscillator 10 and a partial reflector Mm and a total reflector Ms.
The laser beam 1 going out of the laser oscillator 10 is divided into two beams through the partial reflector Mm, and one of the beams passes therethrough to the workpiece 3a so as to remove a coating substance such as the galvanized layer 3b. The other beam is reflected by the partial reflector Mm toward the total reflector Ms and further reflected by the total reflector Ms toward the workpiece 3a, thereby welding it. Thus, as the device moves the beams in a direction 101, it simultaneously carries out removing the coating substance of the work as well as welding.
A Japanese Patent Publication (Kokai) No. 63-112088 discloses welding method of a galvanized iron sheet that has a step for peeling zinc at a surface of a work, as shown in FIG. 24, and a welding step, as shown in FIG. 25.
As shown in FIG. 24, first, the machining head 7 is placed at a position above a normal machining position so that a focus position is located above the surface of the workpiece 3a. Moreover, an output power is decreased, then the laser beam 1 is radiated onto the galvanized layer 3b and moved in the direction 101 along layer 3b in order to peel it off so as to prepare a bare surface 3d of the workpiece 3a for after processing. Next, as shown in FIG. 25, the focus position is set nearer to the surface of the workpiece 3a, then the output power is increased to do welding work.
However, with the laser beam machining of FIG. 21 that uses the mixed gas as an assist gas, quality of the welding decreases if the thickness of the galvanized layer on the surface of the work is large. Moreover, oxygen gas of high purity is generally used as an assist gas in cutting work, so that, if the mixed gas is used also in the cutting work, machining capability thereof is very lowered. Therefore, there is a need to provide a welding method of a galvanized iron sheet having a thick galvanized layer.
FIG. 26 is a cross sectional view showing a welding bead 3e in but-welding a galvanized iron sheet having a galvanized layer thickness of 200 .mu.m using a mixed gas as an assist gas. In the figure, blowholes 3f are produced in the welding bead 3e.
FIG. 22 shows a cause of deterioration in machining quality in a laser beam cutting of the galvanized iron sheet.
Referring to FIG. 22, as a workpiece 3a, comprising a galvanized iron sheet, is cut, the workpiece 3a is formed with a cut groove 3c. A vapor 11, comprising primarily zinc because of its low melted point, intrudes into the cut groove 3c.
As shown in FIG. 22, in the laser cutting of the galvanized iron sheet, the galvanized layer 3b is evaporated and the zinc vapor 11 goes into the cut groove 3c. Accordingly, the purity of oxygen gas in the cut groove 3c is lowered, causing large chips or flaws on a cut surface and drosses at a back surface of the work 3a. Clearly, the laser cutting should be done while preventing the zinc vapor 11 from intruding into the cutting groove 3c.
In the laser beam machining of FIG. 23, the workpiece 3a is welded by a second laser beam after the galvanized layer 3b is peeled off by a final laser beam, so that the welding as a main machining is carried out before the heat generated at the time of peeling of the coating layer 3b is cooled down. If the main machining is done when the machined part is kept heated, heat input is excessive, so that there arises a state of self-burning in the cutting work and a structure of jointed parts is hypertrophied and embrittled in the welding work. In case the positional relationship of the partial reflector Mm and the total reflector Ms is fixed, it is possible that the locuses of the laser beams which performing the peeling operation as pretreatment and the main machining do not coincide with each other if the locuses are made complicated. In case of adopting such a order to make the locuses coincide with each other, the device becomes complicated, expensive and of no practical use. Moreover, the device using the partial reflector Mm and the total reflector Ms is only applicable to linear machining.
In the art shown in FIGS. 24 and 25, since the output power is lowered and the focus is shifted as a machining condition to incinerate and remove galvanized zinc in the first step, the removing speed decreases. Though it is applicable to a galvanizing made of zinc that has a large absorption factor and a low melting point, it cannot be applied to a substance that has a low absorption factor and a high melting point. Moreover, it cannot flexibly deal with the removal of coating materials having different properties. Furthermore, it is necessary to accurately select the output power and speed and control the energy density of the laser beam under a high speed machining, according to a kind of the coating material, in order to properly incinerate and remove the coating material.
In addition, most soft steel materials have an oxide film produced on their surfaces during a rolling process in their manufacture. This oxide film is called a mill scale. In a laser beam machining of a soft steel material that has a thick mill scale layer or a thick and thin mottled layer, dispersion is induced. Namely, if the soft steel material with an oxide film of nonuniform thickness is cut, there is a variation in the absorption factor of the laser beam at the surface of the material. If the absorption factor varies widely, the workpiece quality during cutting is deteriorated. Also in welding the workpiece, if the absorption factor varies, a depth of penetration varies as well, so that a stable machining is impossible. If the welded part (welding bead) is contaminated with the oxide film of the work surface, welding strength is lowered.
Moreover, if the oxide film becomes greater than a certain thickness, cracks are discontinuously produced by thermal shock in laser beam machining, so that the oxide film may exist at one position of the laser beam, and may not exist at another position. As a result, if the soft steel material with the oxide film of nonuniform thickness is cut, there is a variation in the absorption factor of the laser beam at the surface of the material, as described above. If the absorption factor is varied, the work quality in cutting is deteriorated. Also in welding work, if the absorption factor varies, a depth of penetration also varies, so that stable machining is impossible.
Furthermore, rust is produced on the soft steel material if it is left in a high humidity environment for a long time. In laser cutting the part of material that is covered by rust there arises an abnormal combustion and the work quality is deteriorated. Also in laser beam welding of the rusted part, blowholes are produced in the welding bead. Namely, if the work surface is rusted, there is a variation in the absorption factor of the laser beam between rusted parts and non-rusted part. If the absorption factor varies, the work quality in cutting is affected. Also in welding, if the absorption factor varies, the width or depth of penetration varies, so that stable machining is impossible.
The larger work surface irregularity or surface roughness is, the more defects are caused in the work quality in the laser beam cutting.
The larger the surface roughness is, the more that a variation is caused in the absorption factor of the laser beam. The working speed is higher at a part that has a large surface roughness and absorption factor while the working speed is slower at a part that has a small surface roughness and absorption factor, so that work quality is not uniform. Unless the work surface is perpendicular to the radiating direction of the laser beam, flow of the assist gas is disturbed in the cutting groove, thereby causing defective cutting.
Moreover, in the laser beam welding of a high reflectivity material, the laser beam is reflected in machining, so that stable machining work is not possible.